Description:

Now we want to give it a highlight to make our button look like a button. Create a new layer [Icon] with a fill type of transparency.

Call it "highlight". Press CTRL+A to select all, then go Select > Shrink and shrink by 3px. Now choose the rectangular select tool (R) and change the mode to subtract [Selection mode] in the tool options.

Position the mouse cursor at the bottom left corner of the image then click and drag across to the right going half way up the button. You should now have a slim selection that spans the upper length of the image. More detail...